1. Field of the Invention
The present invention relates to a brake hydraulic pressure control apparatus for use with an anti-lock control for preventing the locking of vehicle wheels during braking and for traction control for preventing the slipping of driving wheels during starting or acceleration of the vehicle.
2. Description of the Prior Art
In an anti-lock control apparatus for preventing the locking of wheels during braking of a vehicle to assure security of steering properties and running stability of the vehicle and for reducing braking distance, the control modes of the apparatus (i.e., a pressurization mode, a pressure reducing mode, and a hold mode) for controlling brake hydraulic pressure area determined based on an electric signal expressing the wheel speed obtained from the wheels and speed sensors, so that the brake hydraulic pressure is controlled by a micro-computer by opening or closing hold valves (i.e., pressurizing/holding valves) and decay valves (i.e., pressure reducing valves), which are constituted by electromagnetic valves, in accordance with the control modes so as to pressurize, maintain or reduce the brake hydraulic pressure.
In general, a brake hydraulic pressure control apparatus in which a hydraulic control portion (modulator) for performing the aforementioned anti-lock control is combined integrally with a master cylinder. In the apparatus of this type, as disclosed in U.S. Pat. No. 4,641,895, the master cylinder has primary and secondary pistons arranged in series tandem to each other, so that hydraulic pressures for two braking systems are controlled by the two pistons. The apparatus further has normally opened hold valves provided in hydraulic passages connected between two hydraulic pressure chambers and wheel cylinders, the hydraulic pressures of the chambers being controlled by the two pistons, and normally closed decay valves provided in hydraulic passages connecting the wheel cylinders with a reservoir.
In such a configuration, the hydraulic pressures in the respective hydraulic pressure chambers of the master cylinder are transmitted to the wheel cylinders through the hold valves upon depression of a brake pedal, so that the hydraulic pressures in the wheel cylinders increase to apply braking force to the wheels. When anti-lock control is started, the hold valves are closed to maintain the brake hydraulic pressures, and further, the decay valves are opened when the hold valves are closed. As a result, the brake liquid in the wheel cylinders escapes into the reservoir through the decay valves, so that the hydraulic pressures in the wheel cylinders are reduced to reduce the braking force. On the other hand, at the time of pressurization in anti-lock control, the hold valves are opened, and at the same time, the pistons are operated by high-pressure brake fluids supplied from a hydraulic pressure source such as an accumulator, to thereby increase hydraulic pressures in the wheel cylinders.
However, a shortage of braking force occurs in the case where a failure arises in the hydraulic pressure source systems, such as a hydraulic pressure pump. To compensate for the shortage of braking force, further depression of the brake pedal is required. There arises a problem in that the depth of the brake pedal required for depressing the pedal cannot be obtained.
The applicant of the present application has proposed a brake hydraulic pressure control apparatus as in the specification of commonly assigned U.S. Pat. application Ser. No. 07/261,201 now U.S. Pat. No. 4,867,509 to solve the aforementioned problem. The proposed apparatus comprises: hydraulic passages for connecting the hydraulic pressure chambers of the master cylinder and the accumulator; a normally closed supply valve provided in the middle of the hydraulic passages and operated so as to be opened only at the time of anti-lock control or traction control; intake valves provided in opening portions of the hydraulic passages leading into the hydraulic pressure chambers, the intake valves having forward end portions which do not project into the hydraulic pressure chambers ordinarily but project into the hydraulic pressure chambers only at the time of traction control to close the opening portions to thereby interrupt the hydraulic passages; and valve operating members provided in the pistons of the master cylinder to move together with the pistons to engage with the forward end portions of the intake valves when the stroke of the piston reaches a predetermined value, to thereby open the intake valves.
According to the proposed brake hydraulic pressure control apparatus, the valve operating members open the intake valves as the pistons of the master cylinder move. Accordingly, high-pressure brake fluid is fed into the hydraulic pressure chambers of the master cylinder during anti-lock or traction control when the supply valves are open. As a result, the pistons are returned to set positions and maintained in the set position. Accordingly, the depth of the brake pedal sufficient for depression can be secured in the case where some failure arises in the hydraulic pressure source systems. Accordingly, hydraulic pressures proportional to the force of depressing the brake pedal are fed to the wheel cylinders to attain the necessary braking force.
In the brake hydraulic pressure control apparatus having the aforementioned configuration, a hydraulic passage leading from the downstream side of the supply valve into the reservoir is provided and, further, an anti-lock control normally opened electromagnetic valve is provided in the hydraulic passage so that the hydraulic passage leading into the reservoir is cut off by closing the electromagnetic valve during anti-lock control to feed hydraulic pressure from the accumulator into the hydraulic pressure chambers of the master cylinder through the intake valves.
However, bolts, clips or the like must be used for attaching the anti-lock control electromagnetic valve to the master cylinder. Furthermore, pipings for connecting the electromagnetic valves and the hydraulic passages of the master cylinder are required. Accordingly, not only are the number of parts increased but both additional labor and space for attachment are required.
In addition to the aforementioned arrangement, the brake hydraulic pressure control apparatus described in the specification of the commonly assigned U.S. Pat. application Ser. No. 07/261,201 has a traction control auxiliary hydraulic pressure chamber provided with a sleeve-shaped auxiliary piston at one end of the master cylinder, so that the downstream side of the anti-lock control electromagnetic valve is not connected to the reservoir but is connected to the auxiliary hydraulic pressure chamber. At the time of traction control, the hydraulic pressure from the accumulator is fed to the auxiliary hydraulic pressure chamber to operate the auxiliary piston to thereby operate the pistons of the master cylinder.
However, in the case where maximum necessary hydraulic pressure at the time of traction control is changed, either an interchange of the whole master cylinder or the addition of new parts are required.